The goal of the proposed studies is to learn how Ly-1 lineage B cells are stimulated to proliferate and to elucidate the intracellular mechanisms responsible for S phase entry in these cells. (1) In the first specific aim we will extend our previous work showing that Ly-1+ B cells are uniquely stimulated to initiate DNA synthesis by phorbol ester, without a co-mitogen, by more fully exploring the nature of the phorbol ester responsive B cell. The phenotype of responsive B cells will be examined and the issue of pre-activation will be addressed through evaluation of markers that are signs of B cell activation. (2) In the second specific aim we will explore the intracellular mechanisms that are responsible for phorbol ester induced S phase entry in Ly-1+ B cells. Preliminary results suggest a central role for protein kinase C. We will evaluate PKC activity and isozymes with particular attention to difference between phorbol ester responsive (Ly-1+) and nonresponsive (conventional) B cells. We will then begin to link PKC activity with "downstream" events by determining the stimulated expression of 3 early response gene--c-myc, c-fos, and c-jun-- and by characterizing Fos associated proteins that may regulate gene expression and our finding that Ly-1+ B cells fail to be stimulated through sIg by identifying the level at which sLg mediated signaling is subverted. We will evaluate second messenger generation by measuring production of IP3 and DAG and translocation of PKC, and we will examine the more distal events of early response gene expression. Ly-1+ B cells produce autoantibodies and are implicated in the pathogenesis of autoimmune diseases; ly-1+ B cells readily undergo neoplastic transformation and represent the cell of origin for many B cell tumors. Elucidation of the mechanisms that govern Ly-1+ B cells proliferation may aid in understanding the origin of autoimmune and malignant diseases of lymphocytes and suggest means of influencing their outcomes.